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UBC Theses and Dissertations

Biomass/fossil fuel co-gasification with and without integrated CO2 capture Masnadi-Shirazi, Mohammad Sadegh


Biomass/fossil fuel co-gasification could be a bridge between energy production based on fossil fuels and energy production based on renewable fuels. In this work, CO₂ co-gasification of switchgrass with coal and fluid coke was conducted in a thermogravimetric analyzer at 800°C. Coal gasification kinetics were inhibited or enhanced, depending on the switchgrass concentration in the mixture: for low K/Al and K/Si molar ratios in the mixture, it seemed that the coal ash sequestrated the biomass potassium needed for Kalsilite (KAlSiO4) formation, and thus, no catalytic effect was observed until the biomass to coal mass ratio reached 3:1, where the switchgrass ash supplied enough potassium to more than satisfy the minerals in the coal ash. For high K/Al and K/Si molar ratios in the mixture, the non-reacted residual potassium acted as catalyst and enhanced the coal gasification. The fluid coke contained much lower Al and Si relative to the coal. Hence, the CO₂ gasification kinetics of fluid coke were significantly augmented by blending it with switchgrass, due to the rich presence of potassium in the biomass. A low-ash coal and switchgrass rich in potassium were selected to steam gasify it as a single-fuel and in 50:50 wt% coal:switchgrass mixtures in a pilot scale fluidized bed with silica sand as the bed material at ~ 800 and 860°C and 1 atm. With biomass added to the coal, the hydrogen and cold gas efficiencies, gas yield and HHV of the product gas were enhanced relative to single-fuel gasification. The product gas tar yield was also reduced considerably due to decomposition of tar catalyzed by switchgrass alkali and alkaline earth metals. In the quest for a more sustainable process, coal/switchgrass steam co-gasification was integrated with in-site CO2 capture with limestone. Five gasification/carbonation and calcination cycles were performed in a pilot scale fluidized bed. Hydrogen production was enhanced due to partial adsorption of CO₂ by the CaO sorbent particles (bed material). The sorbent particles decayed and lost their utilization efficiency in the course of cycling due to sintering. A simple equilibrium model and an empirically kinetically-modified equilibrium model were also presented to predict syngas composition.

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Attribution-NonCommercial-NoDerivs 2.5 Canada